Sound is essential for observers to accurately assess unsuccessful conversations.

Perceiving success in group conversations is central to social life, yet little is known about the extent to which the conversation success perceived by conversational partners aligns with that perceived by third-party observers. This study explores how third-party observers perceive conversation success across three sensory conditions (auditory-visual (AV), visual-only (V), and auditory-only (A)) to identify cues of which sensory domain signal perception of success. In two online experiments, older adults (57.1 years (SD = 6.1), N = 160 per experiment) watched and/or heard audio of 2-minute excerpts of previously recorded four-person conversations. Just as the original interlocutors had done themselves, observers provided continuous judgements of conversation success using a slider and an overall success rating after the conversation. Observers judged conversations in AV and V conditions (Experiment 1) and AV and A conditions (Experiment 2). Across both experiments, observers' overall ratings generally aligned with interlocutors' overall success ratings, showing that success can be assessed without active participation. However, for less successful conversations, observers overestimated success when deprived of auditory information (V condition) but not when deprived of visual information (A condition). These findings suggest that auditory cues provide the most critical information for assessing conversation failure, while visual cues alone may mislead observers.